This invention relates to a pilot multiplexing method and OFDM receiving method in an OFDM system and, more particularly, to a pilot multiplexing method and OFDM receiving method in an OFDM system for spreading pilot symbols by prescribed orthogonal codes and transmitting the pilot symbols together with transmit symbols.
Multicarrier modulation schemes have become the focus of attention as next-generation mobile communication schemes. Using multicarrier modulation not only makes it possible to implement wideband, high-speed data transmission but also enables the effects of frequency-selective fading to be mitigated by narrowing the band of each subcarrier. Further, using orthogonal frequency division multiplexing not only makes it possible to raise the efficiency of frequency utilization but also enables the effects of inter-symbol interference to be eliminated by providing a guard interval for every OFDM symbol.
(a) of FIG. 16 is a diagram useful in describing a multicarrier transmission scheme. A serial/parallel converter 1 converts serial data to parallel data and inputs the parallel data to orthogonal modulators 3a to 3d via low-pass filters 2a to 2d, respectively. In the Figure, the conversion is to parallel data comprising four symbols S1 to S4. Each symbol includes an in-phase component and a quadrature component. The orthogonal modulators 3a to 3d subject each of the symbols to orthogonal modulation by subcarriers having frequencies f1 to f4 illustrated in (b) of FIG. 16, a combiner 4 combines the orthogonally modulated signals and a transmitter (not shown) up-converts the combined signal to a high-frequency signal and then transmits the high-frequency signal. With the multicarrier transmission scheme, the frequencies are arranged, as shown at (b) of FIG. 16, in such a manner that the spectrums will not overlap in order to satisfy the orthogonality of the subcarriers.
In orthogonal frequency division multiplexing, frequency spacing is arranged so as to null the correlation between a modulation band signal transmitted by an nth subcarrier of a multicarrier transmission and a modulation band signal transmitted by an (n+1)th subcarrier. (a) of FIG. 17 is a diagram of the structure of a transmitting apparatus that relies upon the orthogonal frequency division multiplexing scheme. A serial/parallel converter 5 converts serial data to parallel data comprising a plurality of symbols (I+jQ, which is a complex number). An IDFT (Inverse Discrete Fourier Transform) 6, which is for the purpose of transmitting the symbols as subcarriers having a frequency spacing shown in (b) of FIG. 17, applies an inverse discrete Fourier transform to the frequency data to effect a conversion to time data, and inputs the real and imaginary parts to an orthogonal modulator 8 through low-pass filters 7a, 7b. The orthogonal modulator 8 subjects the input data to orthogonal modulation, and a transmitter (not shown) up-converts the modulated signal to a high-frequency signal. In accordance with orthogonal frequency division multiplexing, a frequency placement of the kind shown in (b) of FIG. 17 becomes possible, thereby enabling an improvement in the efficiency with which frequency is utilized.
In recent years, there has been extensive research in multicarrier CDMA schemes (MD-CDMA) and application thereof to next-generation wideband mobile communications is being studied. With MC-CDMA, partitioning into a plurality of subcarriers is achieved by serial-to-parallel conversion of transmit data and spreading of orthogonal codes in the frequency domain. Owing to frequency-selective fading, subcarriers distanced by their frequency spacing experience independent fading on an individual basis. Accordingly, by causing code-spread subcarrier signals to be distributed along the frequency axis by frequency interleaving, a despread signal can acquire frequency-diversity gain.
An orthogonal frequency/code division multiple access (OFDM/CDMA) scheme, which is a combination of OFDM and MC-CDMA, also is being studied. This is a scheme in which a signal, which has been divided into subcarriers by MC-CDMA, is subjected to orthogonal frequency multiplexing to raise the efficiency of frequency utilization.
A CDMA (Code Division Multiple Access) scheme multiplies transmit data having a bit cycle Ts by spreading codes C1 to CN of chip cycle Tc using a multiplier 9, as shown in FIG. 18, modulates the result of multiplication and transmits the modulated signal. Owing to such multiplication, a 2/Ts narrow-band signal NM can be spread-spectrum modulated to a 2/Tc wideband signal DS and transmitted, as shown in FIG. 19. Here Ts/Tc is the spreading factor and, in the illustrated example, is the code length N of the spreading code. In accordance with this CDMA transmission scheme, an advantage acquired is that an interference signal can be reduced to 1/N.
According to the principles of multicarrier CDMA, N-number of items of copy data are created from a single item of transmit data D, as shown in FIG. 20, the items of copy data are multiplied individually by respective ones of codes C1 to CN, which are spreading codes (orthogonal codes), using multipliers 91 to 9N, respectively, and products DC1 to DCN undergo multicarrier transmission by N-number of subcarriers of frequencies f1 to fN illustrated in (a) of FIG. 21. The foregoing relates to a case where a single item of symbol data undergoes multicarrier transmission. In actuality, however, as will be described later, transmit data is converted to parallel data of M symbols, the M-number of symbols are subjected to the processing shown in FIG. 20, and all results of M×N multiplications undergo multicarrier transmission using M×N subcarriers of frequencies f1 to fNM. Further, orthogonal frequency/code division multiple access can be achieved by using subcarriers having the frequency placement shown in (b) of FIG. 21.
FIG. 22 is a diagram illustrating the structure on the transmitting side of MC-CDMA (namely the structure of a base station). A data modulator 11 modulates transmit data of a user and converts it to a complex baseband signal (symbol) having an in-phase component and a quadrature component. A time multiplexer 12 time-multiplexes the pilot of the complex symbol to the front of the transmit data. A serial/parallel converter 13 converts the input data to parallel data of M symbols, and each symbol is input to a first spreader 14 upon being branched into N paths. The first spreader 14 has M-number of multipliers 141 to 14M. The multipliers 141 to 14M multiply respective ones of the branched symbols individually by codes C1, C2, . . . , CN constituting orthogonal codes and output the resulting signals. The orthogonal codes C1, C2, . . . , CN are Walsh codes that differ for every user. As a result, subcarrier signals S1 to SMN for multicarrier transmission by N×M subcarriers are output from the first spreader 14. That is, the first spreader 14 multiplies the symbols of every parallel sequence by the orthogonal codes, thereby performing spreading in the frequency direction. Next, a second spreader 15 further multiplies the subcarrier signals S1 to SMN by channel identification codes (cell scramble codes) G1 to GMN and outputs subcarrier signals S1′ to SMN′.
A code multiplexer 16 code-multiplexes the subcarrier signals generated as set forth above and the subcarrier signals of other users generated through a similar method. That is, for every subcarrier, the code multiplexer 16 combines the subcarrier signals of a plurality of users conforming to the subcarriers and outputs the result. An IFFT (Inverse Fast Fourier Transform) unit 17 applies an IFFT (Inverse Fast Fourier Transform) to the subcarrier signals that enter in parallel, thereby effecting a conversion to an OFDM signal (a real-part signal and an imaginary-part signal) on the time axis. A guard-interval insertion unit 18 inserts a guard interval into the OFDM signal, an orthogonal modulator applies orthogonal modulation to the OFDM signal into which the guard interval has been inserted, and a radio transmitter 20 up-converts the signal to a radio frequency, applies high-frequency amplification and transmits the resulting signal from an antenna.
The total number of subcarriers is (spreading factor N)×(number M of parallel sequences). Further, since fading that differs from subcarrier to subcarrier is sustained on the propagation path, a pilot is time-multiplexed onto all subcarriers and it is so arranged that fading compensation can be performed subcarrier by subcarrier on the receiving side. The time-multiplexed pilot is a common pilot that all users employ in channel estimation.
FIG. 23 is a diagram useful in describing a serial-to-parallel conversion. Here a common pilot P has been time-multiplexed to the front of one frame of transmit data. It should be noted that the common pilot P can be dispersed within a frame, as will be described later. If the common pilot per frame is, e.g., 4×M symbols and the transmit data is 28×M symbols, then M symbols of the pilot will be output from the serial/parallel converter 13 as parallel data the first four times, and thereafter M symbols of the transmit data will be output from the serial/parallel converter 13 as parallel data 28 times. As a result, in the period of one frame the pilot can be time-multiplexed into all subcarriers and transmitted. By using this pilot on the receiving side, channel estimation is performed on a per-subcarrier basis and channel compensation (fading compensation) becomes possible.
FIG. 24 is a diagram useful in describing insertion of a guard interval. If an IFFT output signal conforming to M×N subcarrier samples (=1 OFDM symbol) is taken as one unit, then guard-interval insertion signifies copying the tail-end portion of this symbol to the leading-end portion thereof. Inserting a guard interval GI makes it possible to eliminate the effects of inter-symbol interference ascribable to multipath.
FIG. 25 is a diagram showing structure on the receiving side of MC-CDMA. A radio receiver 21 subjects a received multicarrier signal to frequency conversion processing, and an orthogonal demodulator subjects the receive signal to orthogonal demodulation processing. A timing-synchronization/guard-interval removal unit 23 establishes receive-signal timing synchronization, removes the guard interval GI from the receive signal and inputs the result to an FFT (Fast Fourier Transform) unit 24. The FFT unit 24 executes FFT processing and converts the signal in the time domain to N×M-number of subcarrier signals (subcarrier samples) SP1 to SPMN at an FFT window timing. A channel estimation unit 25a performs channel estimation on a per-subcarrier basis using the pilot time-multiplexed on the transmitting side, and a channel compensation unit 25b multiplies the FFT output by channel estimation values CC1 to CCMN of respective ones of the subcarriers.
The channel estimation unit 25a multiplies the subcarrier components of each pilot symbol output from the FFT unit 24 by channel identification Gold codes, adds the results of multiplication subcarrier by subcarrier and calculates the channel estimation values CC1 to CCMN of each of the subcarriers based upon the average value. That is, the channel estimation unit 25a estimates the influence exp(jφ) of fading of each subcarrier on phase using the pilot signal, and the channel compensation unit 25b multiplies the subcarrier signal of the transmit symbol by exp(−jφ) to compensate for fading.
A first despreader 26 multiplies the fading-compensated M×N-number of subcarrier signal components by channel identification Gold codes G1 to GMN and outputs the results. That is, the fading-compensation signals are despread by channel identification Gold codes and a signal of the station's own address is extracted from among the code-multiplexed signals. A second despreader 27 has M-number multipliers 271 to 27M. The multiplier 271 multiplies N-number of subcarrier signals individually by codes C1, C2, . . . , CN constituting orthogonal codes (Walsh codes) assigned to users and outputs the results. The other multipliers also execute similar processing. As a result, the signal addressed to the local station is despread by spreading codes assigned to each of the users, and a signal of a desired user is extracted from the code-multiplexed signals by despreading.
Combiners 281 to 28M each add the N-number of results of multiplication that are output from respective ones of the multipliers 271 to 27M, thereby creating parallel data comprising M-number of symbols. A parallel/serial converter 29 converts this parallel data to serial data, and a data demodulator 30 demodulates the transmit data.
(a) of FIG. 26 is a diagram useful in describing an array of Gold codes G1 to GMN (MN=512) for channel identification purposes. The codes are shifted eight at a time in the subcarrier direction for every OFDM symbol. The reason for shifting the codes is as follows: As mentioned above, the channel estimation unit 25a on the receiving side multiplies the subcarrier components of each pilot symbol output from the FFT unit 24 by channel identification Gold codes, adds the results of multiplication subcarrier by subcarrier and calculates the channel estimation values CC1 to CCMN of each of the subcarriers based upon the average value. That is, a channel estimation value CCm of an mth subcarrier is given by the following equation:CCm=(1/Np)·ΣiRm(k)·*Gm(k) (i=1 to Np)  (1)where Rm(k) represents an FFT output of the mth subcarrier in a kth pilot symbol, Gm(k) a channel identification Gold code of the mth subcarrier in the kth pilot symbol, and * a complex conjugate. Accordingly, in a case where the channel identification Gold codes G1 to GMN are not shifted, as illustrated in (b) of FIG. 26, *Gm(k)=*Gm holds and therefore we have the following:CCm=(*Gm/Np)·ΣiRm(k)  (2)Similarly, if we let Gm(k)′ represent a channel identification Gold code of the mth carrier in the kth pilot symbol of another channel, then a channel estimation value CCm of the mth subcarrier is given by the following equation:CCm=(1/Np)·ΣiRm(k)·*Gm(k)′ (i=1 to Np)  (3)If the channel identification Gold codes G1 to GMN are not shifted, therefore, then *Gm(k)′=*Gm′ holds and therefore we have the following:CCm=(*Gm′/Np)·ΣiRm(k)  (4)If *Gm=*Gm′ holds, then, in view of Equations (2) and (4), the channel estimation values of the mth subcarriers of the two channels will be the same. Moreover, it will no longer be possible to identify which channel estimation value belongs to which channel. For this reason, the channel identification codes are shifted in the subcarrier direction for every OFDM symbol, as depicted in (a) of FIG. 26.
FIG. 27 is a diagram for describing the operation of the channel estimation unit. Here n (n=4) pilot symbols (four OFDM pilot symbols) are multiplexed upon being dispersed within one frame composed of 32 OFDM symbols. Since one pilot symbol is composed of subcarrier samples equivalent to the number M of subcarriers (M×N, e.g., 512), subcarrier-by-subcarrier channel (amplitude characteristic and phase characteristic) estimation becomes possible by multiplying the FFT output by the channel identification Gold codes at the pilot-receive timing on the receiving side. More specifically, to perform channel estimation, n (=4) sets of m (m=8) subcarrier samples in the frequency direction are gathered in the time direction to construct one group by a total of m×n (=32) subcarrier samples, as indicated at PG1 in FIG. 27, and the average value of m×n (=32) channel estimation values in this group is adopted as the channel estimation value of the subcarrier at the center. Further, to obtain the channel estimation value of the next subcarrier, n (=4) sets of m (=8) subcarrier samples shifted by one subcarrier in the frequency direction are gathered in the time direction to construct one group by a total of 32 subcarrier samples, as indicated at PG2, and the channel estimation value is similarly calculated using the average value in the group PG2. The reason for obtaining the channel value by averaging as set forth above is that since each symbol contains noise, the effects of such noise are eliminated by averaging to thereby improve the S/N ratio. If subcarriers are very close in terms of frequency, the channel values are almost the same and therefore no problems are caused by averaging.
In OFDM communication described above, only one pilot can be used and it is not possible to deal with a case where it is desired to use pilots of a plurality of types. For example, as shown in FIG. 28, the vicinity of a base station is divided into sectors and directional beams are emitted from antennas AT1 to AT3 in sectors SC1 to SC3. In this arrangement, it is necessary to identify mobile stations MS1 to MS3 sector by sector. This means that it is necessary to use pilots that differ for every sector. With the conventional methods, however, it is not possible to use pilots of a plurality of types.
A method of using orthogonal codes is available as one method of multiplexing pilot signals of a plurality of types. This method involves adopting m (where m is an integer of one or greater) adjacent subcarriers as a set and multiplying the total of m×n subcarrier components of each set in n-number of pilot symbols by orthogonal codes. Pilot signals the number of which is equivalent to the number of orthogonal codes can be multiplexed at the same frequency and same timing.
If pilots are multiplexed by orthogonal codes, however, demultiplexing of each pilot cannot be performed until all codes are received. Consequently, a problem which arises is that it is not possible to cope with a situation where a momentary channel estimation value is desired at a stage where one or two pilot symbols, for example, have been received.